yes, we have made a few, losing the bottom out of the cardboard box mid race is not fun. A few tips,
you need to use 5mm thick sheet (we use a product called corflute in Oz) 3mm is too wobbly. different colours had different stiffness, black was stiffer than white.
We secured the base and side flaps using contact cement and small bolts and washers, mud flap washers are good for distributing the load. an earlier effort used pop rivets and washers which was also good. the top we held down with velcro and clips. if you make 9 in a day then the contact cement really goes to your head :-)
The downside of our boxes is that they don't collapse down, I have still got two in my backyard as they don't fit anywhere and if you want to holiday as part of the trip you have still got a massive box.
Hopefully the link below takes you through to our third attempt:
We cut out hand holes, then glued a reinforcing plate over them with the grain running perpendicular so that they don't split. then ran gaffa tape around the hole so it doesn't cut your hands.
we had a bunch of velcro straps and the like to hold everything secure on the inside, and some padding below the chain ring and a bracket to pick up the chain stays so that the bike doesn't sit on the derailleur. I also build a little container in to one end to hold skewers, pedals etc. Riding an XL 29er means I need to disassemble a bit more than most people.